Rotary drill bits are commonly used for drilling bore holes or wells in earth formations. One type of rotary drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which typically includes a plurality of cutting elements secured to a face region of a bit body. Generally, the cutting elements of a fixed-cutter type drill bit have either a disk shape or, in some instances, a more elongated, substantially cylindrical shape. A cutting surface comprising a hard, super-abrasive material, such as mutually bound particles of polycrystalline diamond forming a so-called “diamond table,” may be provided on a substantially circular end surface of a substrate of each cutting element. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutting elements or cutters. Typically, the PDC cutting elements are fabricated separately from the bit body and secured within pockets formed in the outer surface of the bit body. A bonding material such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements to the bit body.
The bit body of a rotary drill bit typically is secured to a hardened steel shank having an American Petroleum Institute (API) thread connection for attaching the drill bit to a drill string. The drill string includes tubular pipe and equipment segments coupled end-to-end between the drill bit and other drilling equipment at the surface. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit within the bore hole. Alternatively, the shank of the drill bit may be coupled directly to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit.
Referring to FIG. 1, a conventional fixed-cutter rotary drill bit 10 includes a bit body 12 that has generally radially projecting and longitudinally extending wings or blades 14, which are separated by junk slots 16 extending from channels on the face 20 of the bit body. A plurality of PDC cutters 18 are provided on the blades 14 extending over face 20 of the bit body 12. The face 20 of the bit body 12 includes the surfaces of the blades 14 that are configured to engage the formation being drilled, as well as the exterior surfaces of the bit body 12 within the channels and junk slots 16. The plurality of PDC cutters 18 may be provided along each of the blades 14 within pockets 22 formed in rotationally leading edges thereof, and the PDC cutters 18 may be supported from behind by buttresses 24, which may be integrally formed with the bit body 12.
The drill bit 10 may further include an API threaded connection portion 30 for attaching the drill bit 10 to a drill string (not shown). Furthermore, a longitudinal bore (not shown) extends longitudinally through at least a portion of the bit body 12, and internal fluid passageways (not shown) provide fluid communication between the longitudinal bore and nozzles 32 provided at the face 20 of the bit body 12 and opening onto the channels leading to junk slots 16.
During drilling operations, the drill bit 10 is positioned at the bottom of a well bore hole and rotated while drilling fluid is pumped through the longitudinal bore, the internal fluid passageways, and the nozzles 32 to the face 20 of the bit body 12. As the drill bit 10 is rotated, the PDC cutters 18 scrape across and shear away the underlying earth formation. The formation cuttings mix with and are suspended within the drilling fluid and pass through the junk slots 16 and up through an annular space between the wall of the bore hole and the outer surface of the drill string to the surface of the earth formation.
The bit body 12 of a fixed-cutter rotary drill bit 10 may be formed from steel. Such steel bit bodies are typically fabricated by machining a steel blank (using conventional machining processes including, for example, turning, milling, and drilling) to form the blades 14, junk slots 16, pockets 22, buttresses 24, internal longitudinal bore and fluid passageways (not shown), and other features of the drill bit 10.
FIG. 2 is an enlarged perspective view of a blade 14 showing a plurality of PDC cutters 18 mounted thereon in pockets 22 and supported from behind by buttresses 24. As seen therein, the PDC cutters 18 may include a polycrystalline diamond compact table 36 formed on a substantially planar end surface of a cylindrical substrate 38, the latter being formed of a hard metallic material such as tungsten carbide. Generally, the PDC cutters 18 are secured by their substrates 38 within the pockets 22 by brazing, welding, or adhering using a high-strength adhesive.
In order to enhance the cutting action of the drill bit 10 and/r to prevent wear of drill bit 10, it may be desirable to provide additional “backup” cutters 18′ on one or more blades 14 rotationally behind at least some of the primary PDC cutters 18.
Provision of such backup cutters 18′ in a drill bit 10 that includes a steel bit body 12 may be difficult due to the difficulty of machining pockets 22′ for the backup cutters 18′ using conventional machining equipment (such as, for example, a multiple-axis milling machine) and techniques due to interference between the machining equipment or the cutting element thereof and other features of the drill bit 10 such as, for example, adjacent blades 14. Stated another way, interference between the machining equipment and the drill bit 10 may preclude positioning of the machining equipment and, in particular, the cutting element thereof, in a manner that allows machining of the pockets 22′ for the backup cutters 18′. Furthermore, it may be difficult to machine the pockets 22′ for backup cutters 18′ without machining other areas of the drill bit 10 that are not intended to be machined.
U.S. Pat. No. 7,070,011 to Sherwood, Jr., et al. discloses steel body rotary drill bits having primary cutting elements that are disposed in cutter pocket recesses that are partially defined by cutter support elements. The support elements are affixed to the steel body during fabrication of the drill bits. At least a portion of the body of each cutting element is secured to a surface of the steel bit body, and at least another portion of the body of each cutting element matingly engages a surface of one of the support elements. U.S. Pat. No. 7,070,011 does not describe, teach, or suggest, however, using the support elements disclosed therein to secure backup cutters to a rotary drill bit having a steel body.
Therefore, there is a need in the art for methods that facilitate placement of backup cutters on rotary drill bits, and for rotary drill bits including backup cutters.